A nuclear enzyme, poly(ADPR) synthetase, attaches chains of ADP-ribose to chromosomal proteins. This process occurs maximally in cells at the G2 stage. Two compounds which arrest cells in G2, 5-methylnicotinamide and alpha-picolinic acid, have opposite effects on poly(ADPR) levels of cells. The former reduces the levels by directly inhibiting the synthetase enzyme. The latter compound raises the poly(ADPR) levels by arresting the cells in G2 at a stage in which a) more of the synthetase exists in an active form and b) more of the polymer degrading enzyme exists in an inactive form. It appears that the poly ADP-ribosylation of chromosomal proteins is controlled by processes which actively or passively modify chromatin conformation and therefore these polymer-protein complexes may be intermediates in chromatin strutural rearrangements.